The present invention relates to a writable write-once type optical recording medium, in which data can be written, in particular, having stable record properties and a superior resistance to environment after being recorded and also relates to a method of writing data in the optical recording medium.
The conventional optical recording medium, so-called write-once type recording medium, in which data can be written has a record film containing an organic coloring matter such as cyanines or phthalocyanines, which is well known.
When data is written in such an optical recording medium, laser beams are focused on a very small area of the surface of the record film and converted to thermal energy, thereby characteristics of the record film being changed to form pits. In order to smoothly change the property of the record film, the optical recording medium has in general an air-sandwich structure, which has two substrates as base plates each having one surface coated with a record film, and the substrates are arranged so that the recording films are faced to each other. The substrates are formed of a transparent material.
When data is written to the optical recording medium having such structure, the laser beams for writing are irradiated to the record film through the transparent substrate to form in the record film a pit which can be read by a light. When the written data is reproduced and read, laser beams for reading having lower output than that of the laser beams for writing is irradiated to the written record film to read, as an electric signal, the contrast between the portion where a pit is formed and that where no pit is formed.
There is an optical recording medium in which data has previously recorded, that is, ROM (read only memory) type, which is used in the field of voice recording and information processing. This type of the optical recording medium comprises a plastic substrate, a reflection layer on the substrate and a protection layer on the reflection layer, and prepits, which correspond to data to be reproduced, had been already formed on the plastic base plate by injection molding. The reflection layer is formed of a metal such as Au, Ag, Cu or Al. The typical medium of this type is a compact disk, which is referred to as CD. The specification of signal for recording and reading of the compact disk is standardized, and a device for reproducing the compact disk according to the standard is widely used as a compact disk player (a CD player).
The write-once type optical recording medium in which data can be written is the same as the compact disk in the point of using laser beams as a light for reproducing data and having a disk form. Therefore, there is an extensive development of the optical recording medium in which data can be written, which is adapted to the standard of the specification of the compact disk and can be used in the compact disk player as it is, that is, having a superior player ability.
In the development, there has been studied the write-once type optical recording medium in which data can be written, comprising a light transmissive substrate, a record film containing an organic coloring matter which is formed on the substrate, a light-reflection film which is formed on the record film and a protection film which is formed on the light-reflection film.
However, in this type of the optical recording medium, data can be written, for example, in accordance with the following principle. The coloring matter in the record film absorbs the irradiated light and generates thermal energy. The record film and the base plate are deformed and changed in quality by the thermal energy to form a pit.
The protection film is provided for the purpose of preventing the degradation of the record film and the reflection film, and preventing cracks from forming in the optical recording medium in which data can be written. It was, however, proven that, if the record film contains phthalocyanine as a coloring matter, the signal recording characteristics depend on the thickness of the protection film, and normal reproduction becomes impossible when the thickness of the protection film is less than a certain thickness. In detail, in the optical recording medium having a record film containing phthalocyanine, the deformation on the formation of a pit occurs not only in the direction of the substrate but also in the directions of the light-reflection film and the protection film, in contrast with the optical recording medium having a record film containing cyanine as a coloring matter, so that, when the thickness of the protection film is less than a certain thickness, the recording characteristics become deteriorated and if the thickness of the protection film is furthermore decreased, the light-reflection film and the protection film are partly destroyed and normal reproduction becomes impossible.
The pit formed in the optical recording medium in which data can be written is different in a form from that in the read only memory (ROM) type of the optical recording medium, so that the playerability of the optical recording medium in which data can be written for the compact disk player already prevailed is insufficient. The optical recording medium after being recorded as described above has not a sufficient resistance to environment Such as light, temperature and moisture.